The present invention relates to cementing operations and, more particularly, to plug-and-abandon operations. In the process of drilling and completing hydrocarbon wells, it is common place to use heavy steel casing in a well and to place cement between the casing and the borehole to anchor the casing in place and prevent migration of fluids along the annulus outside the casing. After an upper portion of a well has been drilled and casing is cemented in place, it is common to resume drilling the well and install a liner in the lower part of the well by lowering the liner through the upper-cased portion of the well. Liner hangers are used to mechanically support the upper end of the liner, typically at the lower end of the previously set casing, and to seal the liner to the casing. Traditional liner hangers include slips for mechanical support of the liner and packers for forming a seal between liner and casing.
Expandable liner hangers are now commonly used in wellbore operations and provide advantages over traditional methods. These liner hangers are expanded against the wall of the previously set casing, such as those sold under the trade name VERSAFLEX, by Halliburton Energy Services, Inc., have been developed. Expandable liner hangers provide both mechanical support and a fluid seal by use of a number of annular seals, typically elastomeric rings, carried on the exterior of the expandable liner. In operation, the liner hanger is positioned in a cased portion of a well, and an expansion device is forced through the liner hanger to radially expand the liner hanger toward or into the casing wall, compressing the elastomeric seals to provide both mechanical, bidirectional support and a fluid seal.
At the conclusion of the life of the well, the well must be plugged and abandoned. In performing plug-and-abandon operations, a plugging composition (e.g., Portland cement, kiln dust, fly ash, slag cement, shale, etc.) is placed in the well at a desired depth. The plugging cement is pumped or circulated into the well, where it sets, forming a hardened mass (e.g., a plug) that seals off selected intervals of the well. The plug prevents or reduces zonal communication and migration of fluids that may contaminate water formations. This also prevents the migration of gas or fluids to the surface. It may be desirable to form plugs adjacent to hydrocarbon-producing formations and water-containing formations.
The number of hydrocarbon fields approaching the end of their lifespan is rapidly increasing. It is estimated that approximately 30,000 wells worldwide will have to undergo plugging and abandonment within the next fifteen years. Approximately 30% of these wells are subsea wells. In the North Sea alone, for example, it is estimated that about 6,000 wells will be subjected to plug and abandonment including 1,400 subsea wells, depending on aging profiles. In Asia, the average subsea well is over five years old. In Malaysia, 70-80% of the subsea wells are mature and on average 20-25 years old. Wells passing maturity in the Gulf of Mexico is also rapidly growing.
With the increase in concern and regulation regarding potential environmental impact of abandoned wells, improved methods and apparatus are desired for plugging and abandoning wells and insuring the integrity of the plugs. In some instances, plugs are compromised or casing deflected after completion of the P&A operation. Traditional liner hangers, mechanical packers, and the like, tend to have little ability to deform in response to these changes and practically no ability to further autonomously expand after the initial setting.
Traditional methods of plugging and abandoning subsea wells require use of expensive and slower heavy-duty and conventional rigs. It is desirable, therefore, to plug and abandon wells using lightweight and riserless well intervention with a consequent drop in expense and mobilization times. Further, the fleet of vessels suitable for such operations is larger than the available heavy-duty and conventional fleet. The lightweight and riserless intervention approach addressed herein provides a versatile solution to plug and abandonment operations offshore. It covers all aspects of the operation, from initial logging and inspection to plugging.